On basin hyposmetry and the morphodynamic response of coastal inlet systems

Abstract

The exchange of water and entrained material between coastal basins and the inner continental shelf reduces to the problem of polarized transport (flood +, ebb —) in the conveyance channels routing flow through a coastal inlet. Here the net long-term movement of materials is largely a function of the fluid velocity and discharge — variables whose time history at-a-station contains both periodic and aperiodic elements. In this paper the importance of the major periodic elements in coastal inlet flows and their potential contribution to the net transport of bedload materials is discussed. With the aid of a numerical model featuring a closed hypsometric (area-height) representation of basin storage-volume—channel-flow relationships in a typical basin and inlet system at Wachapreague, Virginia, on the East Coast of the United States were studied to determine present mechanisms for inducing net flood or ebb bedload transport. The basin hypsometry and channel dimensions of the prototype system were then varied in the model to simulate other conditions that may have prevailed during earlier stages of basin evolution. Given a sine wave input to the model, the principal lunar semidiurnal constituent (M 2) and its first-harmonic overtide (M 4) respond in a predictable way to differences in basin hypsometry and channel configuration, and these constituents in turn account for distinctive rise and fall duration differences observed in the mean tide within the basin. These distortions in basin tides are also reflected at-a-station in the time histories of channel discharge and velocity in the form of greater peak magnitudes during ebb or flood depending upon which phase has the shorter duration, and depending in some instances on a further modulation by basin hypsometry and channel cross-sectional area. A consistent imbalance between peak flows of opposing direction is a plausible mechanism for long-term, net transport of bottom sedimentary materials. Thus it appears that a systematic contribution toward either flood or ebb dominance in channel flows can arise through complex periodic tides that exist as a result of specific morphological features in basin and inlet systems. The primary significance of these periodic mechanisms may lie in their apparent sensitivity to changes in basin hypsometry.